This study is a cross-sectional and longitudinal study of the natural history and pathogenesis of MAC disease in a cohort of late-stage HIV-infected subjects and the effect of HAART (Highly Active Antiretroviral Treatment) on DMAC (Disseminated infection with Mycobacterium avium complex) and opportunistic infection relevant immune cell population and functions. The purpose of this study is to determine if tissue infection with DMAC occurs prior to sustained bacteremia by obtaining bone marrow biopsies and blood for quantitative cultures from subjects at high risk for DMAC. Further, we propose that aggressive antiretroviral drug treatment of HIV-infected subjects at risk for DMAC because of specific immune deficiencies will improve these immune functions in such a manner as to resist DMAC.